Method of and means for maintaining a halocline in an open body of salt water

ABSTRACT

A halocline is maintained in open body of salt water at a depth to which a significant amount of solar radiation penetrates by inducing an upward vertical flow in the body of water sufficient to counter wind-mixing and molecular diffusion thereby establishing an ascending or rising solar lake. The upward flow is induced by injecting into the body of water a concentrate with a density greater than the density of liquid at the lower end of the halocline, the concentrate being formed by evaporating liquid drawn from the surface of the body of water. The halocline suppresses convention currents and allows solar radiation to heat the halocline as well as a layer of liquid therebelow to temperatures significantly higher than the surface temperature. Heat for useful work can be extracted from the heat storage layer beneath the halocline. 
     In a modification, the linearity of the halocline and its consequent stability are controlled by inducing a downward vertical flow simultaneously with and equal to the upward flow thereby establishing what is termed a standing solar lake. The downward flow is induced by flash evaporating liquid drawn from the heat storage layer.

This is a division of application Ser. No. 134,658 filed Mar. 27, 1980,now U.S. Pat. No. 4,440,148 which is, in turn, a continuation-in-partapplication of patent application Ser. No. 828,190, filed Aug. 26, 1977,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method of and means for maintaining ahalocine in an open body of salt water.

A halocline is a zone in a body of salt water wherein a marked salinitygradient is present, the gradient being such that salinity increaseswith depth. As is well known, convection currents within a halocline aresuppressed by the difference in vertical density. As a consequence, thepresence of a halocline adjacent a surface of a body of water at a depthto which solar radiation penetrates (i.e., 1-3 meters) is accompanied byheating of the halocline and a layer of liquid therebeneath totemperatures significantly higher than the surface temperature of thebody of water. In the absence of the halocline, heat absorbed below thesurface of the body of water would be transferred by convection currentsto the surface of the water where it would be dissipated by evaporationand long-wave radiation. Thus, a stable halocline in a shallow pondconverts the pond into a solar collector permitting temperatures as highas 100° C. to be obtained at a depth of about one meter below thesurface.

The ability of shallow pond to act as a solar collector depends upon thestability of the halocline. Factors tending to destabilize a pond aremolecular diffusion of salt along the concentration gradient, mixing ofthe upper layer of liquid due to wind action at the surface, and inducedconvection associated with heat extraction at the bottom of the pond.

An initial approach to maintaining stratification and stabilizing thehalocline is disclosed in Israeli Patent No. 12561 of May 25, 1959. Thispatent discloses a system for continuously flushing the surface of asolar pond with fresh water and adding salt at the bottom. In order forthis technique to be successful, a source of fresh water is required andthe pond is not self-maintained in the sense that it requiresintercession from outside the system to maintain the halocline.

A self-maintaining solar pond is disclosed in U.S. Pat. No. 3,372,691 ofMar. 12, 1968 wherein a downward vertical motion across the halocline isinduced by flash evaporating liquid drawn from a heated layer of liquidbelow the halocline to obtain fresh water and a solution whoseconcentration exceeds that of the heated layer liquid. Some of theconcentrated solution is returned to the heated layer of liquid, andmake-up water to maintain the level of the pond is added at its surface.A solar collector operating on this principal is termed a descending orfalling solar pond because of the downward motion across the halocline.Such motion can be adjusted, in theory, to counter the diffusion flux ofsalt in the pond by controlling the rate at which flash evaporationtakes place.

It can be shown analytically that as the downward motion across thehalocline increases, the slope of the halocline in its upper region(i.e., near the surface of the pond), becomes quite large, and in fact,the salinity profile is asympotic to the vertical near the surface. As aconsequence, the salt gradient near the top of the halocline will bevery small and hence highly unstable when heated by more than a fewdegrees. Furthermore, wind mixing near the surface will completelydestroy small salt gradients.

The situation in the upper portion of the halocline can be improved ifmost of the fresh water produced by the flash evaporator is returned tothe pond. However, in arid zones, fresh water is scarce and its use inthis manner is wasteful. Moreover, brackish or sea water is oftenavailable as make-up water to compensate for evaporation, but the saltfrom such water will slowly accumulate and eventually destroy thestratification. In order to maintain the balance, downward movementacross the halocline can be increased to not only compensate formolecular diffusion, but to bring salt to the bottom of the pond whereit is available for crystalization from the flash evaporator. It can beshown that as the salinity of the make-up water increases, an evengreater vertical movement is needed, and such greater vertical movementfurther reduces the already small gradient in the upper half of thehalocline. As a consequence, the pond under these conditions will bevery unstable against wind mixing and induced mixing due to circulationassociated with heat extraction.

It is therefore an object of this invention to provide a new andimproved method of and means for maintaining a halocline in an open bodyof water wherein the disadvantages of the prior art, as outlined above,are reduced or substantially overcome.

SUMMARY OF INVENTION

In accordance with the present invention, a halocline in an open body ofwater is maintained by inducing an upward vertical flow sufficient tocounter wind-mixing and molecular diffusion thereby establishing anascending or rising solar pond. The upward flow is induced by injectinginto the body of water a concentrate with a density greater than thedensity of liquid at the lower end of the halocline, the concentratebeing formed by evaporating liquid drawn from the surface of the body ofthe water. The halocline suppresses vertical convection currents and itslocation near the surface of the body of water enables solar radiationto heat the halocline and the layer of liquid therebelow to temperaturessignificantly higher than the surface temperature. Heat for useful workcan be extracted from the heated layer.

In a second embodiment of the invention, the linearity of the haloclineand its consequent stabilization are controlled by inducing downwardvertical flow simultaneously with, and equal to, the upward flow therebyestablishing what is termed a standing solar pond. The downward flow isinduced by flash evaporating liquid drawn from the heat storage layer.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention are shown in the attached drawings to whichreference is now made:

FIG. 1 is a vertical section taken through an ascending solar lake inaccordance with the present invention;

FIG. 2 is a graph showing the salinity and temperature profiles througha vertical section of the lake;

FIG. 3 is a vertical section taken through a standing solar lake inaccordance with the present invention;

FIG. 4 is a graph showing the variation in salinity in a halocline for astanding, an ascending, and a descending pond; and

FIG. 5 is a perspective view of a wind break usable on the surface of astanding or ascending solar lake;

FIG. 6 is a vertical section taken through a solar pond showing acontrol chamber that functions to maintain the gradient without thenecessity of an evaporating pond;

FIG. 7 is the salinity profile of the pond;

FIG. 8 is a vertical section taken through a solar pond showing a flashevaporator that eliminates the need for pumps to transfer brine.

FIG. 9 is a graph showing heat extraction capabilities for a standing,ascending, and falling solar pond.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, reference numeral 10 designates an ascending orrising solar lake according to the present invention wherein halocline12 in open body 14 of relatively deep salt water is maintained byinducing an upward vertical flow through the halocline. The verticalsalinity distribution through the body 14 is shown at 16 in FIG. 2. Ifthe origin of the vertical axis is taken at the depth of the lower endof the halocline, the latter is a layer of thickness h located below alayer of thickness D-h defining surface 18 and constituting a mixedlayer 20 whose salinity is relatively low and substantially uniform dueto wind mixing. Within the halocline, the salinity increases sharplywith depth as shown in FIG. 2; and deeper than the halocline, thesalinity is substantially constant with depth. Quantatively, thedistance D, is from one to three meters and the distance D-h, will beabout 30 cm, wind breaks shown in FIG. 5 being floated on surface 18 toinsure the situation.

Solar radiation 22 incident on surface 18 will penetrate and be absorbedin both the mixed layer 20 and the halocline. By reason of wind-mixingin the mixed layer, heat absorbed in this layer will be dissipatedquickly at the surface thereby maintaining a relatively low temperatureas compared to the temperatures in the halocline wherein convectioncurrents are suppressed. As a consequence, the halocline will be heatedto temperatures substantially greater than the temperature above thehalocline as indicated in temperature profile 24 shown in FIG. 2. If thehalocline is established over a period of time, the liquid below thehalocline will be heated by conduction and convection currents thereinto establish a heat-storage layer 26. In the hypolimnion 28 below layer26, the temperature will be considerably lower as indicated in FIG. 2.

In the preferred form of the embodiment shown in FIG. 1, liquid from theupper region of layer 26 is drawn through inlet conduit 30 into heatutilization means 32 (e.g., a heat exchanger) which extracts heat foruseful purposes (e.g., driving a turbine which drives an electricgenerator). After the heat is extracted, the cooled liquid is dischargedthrough conduit 34 into the lower portion of layer 26. The thickness ofthe heat storage layer is thus dependent on the location of heatextraction discharge (34). In alternative forms of the invention, theheat utilization means can be in the form of a series of heat exchangersdirectly immersed in heat-storage layer 26.

Solar lake 10 includes a relatively shallow evaporation pond 36 adjacentbody 14 and interconnected therewith by a channel shown schematically at38 which allows surface liquid from body 14 to be drawn into the pondand permits circulation to occur between body 14 an the pond throughconduit 40. Evaporation from pond 36 takes place due to climaticconditions of humidity and solar radiation providing a concentrate inthe pond whose density will exceed the density of the liquid in body 14below the halocline. The concentrate will flow through conduit 40 andinto the body 14 below the halocline. The circulation thus establishedcauses an upwelling in the body 14 that stabilizes the halocline againstboth wind-mixing and molecule diffusion of salt across the halocline.

To maintain the surface 18 at a substantially constant level againstevaporation from both this surface and pond 36, replacement liquid mustbe added. In arid climates, little rain will be available for make-upwater and fresh water will be a premium. Therefore, since brackish orsea water will usually be available as make-up water, the added saltspresent in the make-up water can be recovered as solids in theevaporation pond.

The mechanism for establishing and maintaining the halocline in a largebody of water is applicable to artificial lakes as well as to certainnatural bodies of water. It is discussed below, for illustrativepurposes only, with reference to a proposal for converting the Dead Seainto an ascending solar lake. The material that follows is based on anarticle, which is hereby incorporated by reference, published in SolarEnergy, Vol. 18, pp. 293-299, Pergamon Press, entitled "The Dead Sea: AScheme for a Solar Lake" by the present inventor. The principles arealso applicable, however, to other bodies of water lying in aridclimates, as for example, the Great Salt Lake, the Quantara Depression,and the Gulf of Suez. As applied to the Dead Sea, body 14 represents therelatively deep northern basin of the Sea with an area of 720 Km², andpond 36 represents the relatively shallow southern basin with an area of230 Km². Conduit 38 represents the Lisan Straits that interconnect thetwo basins.

At the present time, evaporation ponds associated with the southernbasic currently draw a flux m_(c) and produce halite precipitates. Theseponds will be flushed with relatively fresh water (m₂ in FIG. 1 having asalinity S₂) and become dissolving ponds. The brine discharge from thehalite production will mix with the discharge from the dissolving pondsproviding the flow m₃ in FIG. 1 whose density will exceed the density ofthe liquid in body 14. In FIG. 1, m₁ represents the mass flow of make-upwater of salinity S₁ and Mv^(P) represent the mass flow of fresh waterevaporated from the body and from the pond respectively.

The parameters by which the halocline can be stabilized are discussedbelow. First of all, the depth D of the halocline should be minimized inorder to locate the halocline as close to the surface as is practical inorder to maximize the amount of heat converted from solar radiation. Theparameter that minimizes D will determine the upwelling rate and thecirculation required to establish such rate. Finally, the resultantupwelling rate must be sufficient to overcome molecular diffusion inorder to provide stability to the halocline.

It can be shown that the depth of the halocline is directly proportionalto Cu, the rate at which wind introduces available energy for mixing,and inversely related to the quantity:

    S.sub.2 (ρ.sub.0 -ρ.sub.2)/(S.sub.0 -S.sub.2)

where ρ₀ and ρ₂ are the densities of liquid at depth D and on thesurface respectively, and S₀ and S₂ are the salinities (in percent) atthe same locations. Obviously, Cu (which depends on the cube of the windspeed) can be minimized by providing floating wind breaks as describedbelow. This will also minimize the thickness of the mixed layer 20 whichis given by the value D-h.

The other quantity upon which D depends must be maximized in order tominimize D. From consideration of stability of liquids involved, thisquantity cannot grow beyond bounds; and for Dean Sea water, the quantityis maximized when the surface salinity S₂ =15%.

It can be shown from kinematic analysis that the upwelling rate W isgiven by: ##EQU1## where E is the evaporation rate of pond 36, A.sub.ρis its area, A₀ is the area of body 14 at the level of the halocline(i.e., Z=0), ρ₀ is the density at this level, and S_(c) is the salinityof the flux m_(c). Thus from equation (1), both the upwelling rate andthe circulation can be determined from the known parameters.

In order for the halocline to be stabilized against molecular diffusion,the upwelling rate must exceed a threshold value which can be determinedfrom the equation representing a steady state salt flux F across thehalocline due to molecular diffusion:

    WS-KS'=F/ρ                                             (2)

where W is the upwelling rate (i.e., the upward velocity of water inbody 14), S is the salinity, K is the salt diffusivity, S' is thevertical rate of change of salinity, and ρ is density.

Integration of equation (2) from Z=0 TO Z=h (see FIG. 1) and along thehalocline yields:

    S(Z)=S.sub.0 +[S.sub.h -S.sub.0 ][1-exp (WZ/K)]/[1-exp (Wh/K)](3)

where S(Z) is the salinity within the halocline at point Z above thebottom of the halocline, S₀ is the salinity at the bottom of thehalocline, and S_(h) is the salinity at the top of the halocline.

Referring now to FIG. 4, Equation (3) has been plotted in normalizedfashion for various values of (Wh)/K. As can be seen, the profile of thesalinity of the halocline will be nonlinear and will bulge upwardly ofan ascending pond. For stability reasons it would be desirable to have(Wh)/K less than three since this condition would cause the profile toapproach linearity.

FIG. 4 also shows the salinity profile in the halocline for a descendingsolar pond of the type disclosed in U.S. Pat. No. 3,372,691. Frominspection of FIG. 4 can be seen that the superposition of an ascendingsolar pond on a descending solar pond should permit the establishment ofa linear salinity profile in the halocline which is a situation thatresults from a net flow of zero. This can be achieved in the mannershown in FIG. 3 to which reference is now made.

In FIG. 3, reference 50 designates a standing solar pond according tothe present invention which comprises a main pond 52 about two metersdeep and a shallower evaporation pond 54. The main and evaporation pondsare interconnected by a conduit 56 such that the surface waters of themain pond can flow into the evaporation pond which is connected byconduit 58 to a region of the main pond below the halocline 60. By theprocess described in connection with the first embodiment, halocline 60is established with a salinity profile indicated at 62. Below thehalocline is a heat storage layer 64 to which flash evaporator 66 isconnected by conduit 68.

In operation, surface water in pond 52 is drawn into pond 54 whereevaporation takes place producing a concentrate which flows throughconduit 58 into the bottom of pond 52 producing an upwelling in themanner described above in connection with the first embodiment of theinvention. Liquid from the heat storage zone is drawn into the flashevaporator which produces a precipitate of salt and fresh water. Theeffect of the flash evaporator is to induce a descending motion in themain pond which cancels the ascending motion due to the injection of theconcentrate from pond 54. As a consequence, the profile of the salinityin the halocline is linear as indicated.

From considerations of the conservation of volume and mass in thestanding solar pond shown in FIG. 3, it can be shown that the density ofthe concentrate required to establish the standing solar pond is asfollows: ##EQU2## where the symbol "q" represents a volume transport (m³/sec), the symbol "ρ" represents density and "σ" the density anomaly(σ=ρ-1000) in (kg/m³), the subscript "e" represents an index ofevaporation, the symbol "i" represents an index of the in-flow to thepond, and the index "f" represents an index of the fresh water outputand "s" the salt output of the flash evaporator. In addition, Δρ=-(ρ_(h)-ρ₀), and h is the depth of the halocline, K is the salt diffusivity,and A is the area of the main pond, it being assumed that the area ofthe evaporator pond 54 is substantially smaller than the area of themain pond.

Referring now to FIG. 5, a wind break suitable for use with eitherembodiment of the invention is shown. The wind break comprises aplurality of members 60 running longitudinally and a plurality ofmembers 61 running transversely and interconnected at their junctions.The height of the wind break is n and the spacing between the windbreaks is 15n. If the wind break has a porosity of 40%, the wind speedat the surface can be reduced to about 1/3 within the distance 15n.

The provision of a wind break on the main pond to reduce wind mixingwill be advantageous when a layer of material is provided on the pondsurface to reduce evaporation. As is well known, the deposition on anarea body of water of a monomolecular layer will significantly reduceevaporation. Many materials can be used, as for example HEXADECANOL. Theproblem with using a monomolecular layer to retard evaporation is itssusceptability to being removed from the central region of the pond bywind action. That is to say, the wind tends to blow the evaporationretardant toward the periphery of the bond thus exposing the centralportion of the pond to evaporation. By providing a wind break,dispersing of the evaporation retardant is resisted and the retardant isthus considerably more effective than would be the case were theretardant used without a wind break.

A deep solar lake (i.e., one of depth greater than 10 m) is advantageousbecause the hypolimnion can be utilized as a heat sink into which heatfrom the power plant can be rejected. That is to say, the hypolimnioncan act as a condenser for the exhaust of a heat engine operating onheat extracted from the heat storage layer. When this occurs, it willusually be necessary to remove some heat from the hypolimnion. Aparticularly simple arrangement to achieve this end is to pump waterfrom the hypolimnion into the evaporation ponds. Because such ponds arerelatively shallow, their surface temperature will be in thermalequilibrium with the atmosphere. Consequently, the heated, butrelatively dense, water from the hypolimnion will be cooled in theevaporation ponds and be available for return to the solar lake as evendenser "deep water" that induces upwelling. Thus, the evaporation pondwill serve as a means for disposing of waste heat as well as a source of"deep water". FIG. 1 shows condenser 31 interposed between hypolimnion28 and evaporation pond 36. Cool water from the hypolimnion passesthrough the condenser, extracting heat from heat utilization means 32;the warmed water from the hypolmnion is then passed to the evaporationpond where the extracted heat is transferred to the atmosphere.

The amount of water from the hypolimnion that must be cooled to maintainequilibrium conditions will not be large. For example, in the Dead Sea,the equilibrium temperature of the Southern Basin is about 17 C. in thewinter and about 35° C. in the summer. If deep water from the NorthernBasin is used for cooling a power station; and if the temperature of thedeep water transferred from the Northern Basin to the Southern Basin is25° C., it will be cooled in winter at the rate of 400 watts/m².

The above discussion deals with processes that automatically maintain ahalocline in a solar pond against salt diffusion and wind mixing byevaporating surface water from the solar pond to a density greater thanthe density in the heat storage layer below the halocline and allowingthe denser water to flow into the heat storage layer at a rate whichproduces an ascending solar pond. Given a predetermined functionalrelationship between the salinity in the halocline and depth, theprocess described above will maintain this relationship over a longperiod of timme. Using an indirect heat exchanger, heat can be extractedfrom the heat storage layer at the same average rate at which the heatstorage layer absorbs heat from the halocline. For example, in thegeographic area in the vicinity of the Gulf of Eilat, the heat flux fromsolar radiation is about 250 watts/m² day and night, the year round. Asolar pond is about 20% efficient, so that the heat flux into the heatstorage layer of a solar pond in this vicinity will be about 50watts/m². If this amount of heat were extracted from the pond, in termsof the latent heat of water, then the pond could furnish water at a rateof about 1.8 Kg/day/m², which is equivalent to evaporating a layer ofwater about 1.8 mm deep. While more heat can be extracted from a solarpond over a short period of time when the pond operates intermittently,the heat flux into the heat storage layer represents the steady staterate at which heat can be extracted from the heat storage layer 24 hoursper day indefinitely, provided only that a mechanism exists formaintaining the predetermined functional relationship between thesalinity and the depth of the halocline.

In addition to the techniques described above, the present inventionincludes other techniques suitable for solar ponds where an auxiliaryevaporating pond is not convenient as, for example, in humid areas. FIG.6 represents one such technique wherein reference numeral 70 designatesa solar pond according to the present invention into which controlchamber 71 is incorporated for automatically maintaining the stabilityof the halocline. Pond 70 includes the usual basin 72 for containing thepond which includes top mixed layer 74 adjacent surface 76 of the pond,halocline 78 just below the mixed layer, and heat storage layer 80 belowthe halocline. The salinity profile of the water in pond 70 outside thecontrol chamber is shown in FIG. 7, the salinity of the mixed layerbeing designated S₁, and the salinity of the heat storage layer, and atthe bottom of the halocline, being designated S_(B).

Chamber 71 defines local region 82 which functions to evaporate waterfrom the pond, thereby increasing the salinity in the region.Specifically, chamber 71 includes tube 84 vertically positioned in thepond, the lower end of the tube being open at 86 at an elevation belowthe halocline. The upper end of the tube is closed by cap 88 and a vent89 in the closed end of the tube provides an exit passage for water thatevaporates from inside the tube. An optional way of mounting the tube inthe solar pond is indicated in FIG. 6 and included floats 87 surroundingthe tube such that the tube floats in the pond with cap 88 and vent 89being located above the surface 76 of the pond. Cables 85 are used tomoor the tube in the pond.

Float 87, which is annular, surrounds the lower portion of the tube andprovides for insulating the water in the tube as well as bouyancy forthe tube.

In operation, water column 81 inside the control chamber issubstantially at the temperature of the heat storage layer and is thuswarmer than the surface waters of the pond. Furthermore, the waterinside the control chamber is more dense than the water in thehalocline, with the result that the surface of the water in the controlchamber is somewhat lower than the surface of the water in the pond. Byreason of the temperature of the water in the control chamber andclimatic conditions, surface water in column 81 will evaporate andperhaps condense inside cap 88, where it will roll down the inclinedsides and exit through openings 89 dripping into the pond outside thecontrol chamber. The evaporation of water in column 81 the controlchamber is accompanied by an increase in salinity and hence density ofthe water in column 81 relative to the density of the water in the heatstorage layer. As a consequence, a natural flow of water into and out ofthe control chamber is set up in the manner shown by the arrows in FIG.6. Specifically, water flows relatively lighter water from the heatstorage layer flows upwardly into column 81 in an annular region ofopening 86, and relatively denser water in the column flows downwardlythrough the central region of the opening.

The walls of tube 84 may be apertured in the region of the mixed layerfor the purpose of adding surface water to the interior of the tube inaccordance with adjustable control 83.

In general, the mass and salt balance is maintained by inputting waterat a rate q_(in) and discharging water at a rate q_(d). Water evaporatesfrom the pond at the rate q_(e) while water vapor escapes from thecontrol chamber in the amount of q_(out). Water at the rate of q₁ fromthe surface is admitted to the interior of the control chamber. Finally,water at the rate q_(u) flows upwardly through the heat storage layer.

By considerations relating to the conservation of volume and salt in thebottom layer of the pond, the bottom layer balance is as follows:

    q.sub.u +q.sub.out =q.sub.1                                (5)

    S.sub.B q.sub.u -(K/h)(S.sub.1 -S.sub.B)=q.sub.1 S.sub.1   (6)

These equations can be solved to yield:

    q.sub.out =[(S.sub.B -S.sub.1)/S.sub.B ][q.sub.1 +q*]      (7)

where q*=K/h, K is the diffusivity of salt, and h is the depth of thehalocline.

Eq. (7) is based on the assumption that the quantity K is constant inthe halocline and that the halocline is linear. These are reasonablyaccurate assumptions and will yield realistic results.

It should be noted that the quantities q is expressed in velocity unitsrelated to the area of the pond. For example, the quantity q* for ahalocline of 1 m is approximately 0.2 mm/day.

Three different regimes can arise depending upon the relationshipbetween the quantity q₁ and the quantity q_(out), namely, an ascendingpond, a falling pond and a standing pond. As ascending pond is achievedwhen q₁ is greater than q_(out) ; a falling pond, when q_(out) isgreater than q₁ ; and a standing pond results when q_(out) =q₁.

Considering first an ascending pond wherein the amount of water fed intothe interior of the control chamber exceeds the amount of waterevaporated from the control chamber, the relationship between these twoquantities may be expressed as q₁ =(1+δ) q₀. When this relationship issubstituted into Eq. (7), the relationship between the amount of waterevaporated and the quantity q* is as follows:

    q.sub.0 /q*=[1-S.sub.1 /S.sub.B ]/[S.sub.1 /S.sub.B)(1+δ)-δ](8)

Eq. (8) is plotted for two parameters in FIG. 9, curve 100 representingthe solution of Eq. (8) when the surface water added to the interior ofthe control chamber exceeds the water evaporated from the controlchamber by 50% while curve 101 represents the situation when the excesswater added is 25% of the water evaporated.

For a falling pond, the amount of water evaporated from the controlchamber will exceed the amount of water which enters the control chamberthrough control 83. The relationship between these two quantities can beexpressed as q₁ =(1-Δ)q₁ and, in such case, Eq. (7) can be rewritten asfollows:

    q.sub.o /q*={[1-(S.sub.1 /S.sub.B)]q*}/[(S.sub.1 /S.sub.B)(1-Δ)+Δ]                             (9)

Eq. (9) is also plotted in FIG. 9 wherein curve 102 represents a fallingpond in which 20% more water is evaporated than is added to the controlchamber; and curve 103 represents a falling pond in which the differenceis 10%.

For a standing pond, the amount of water added to the control chamber isexactly equal to the amount of water evaporated from the controlchamber. The equation for describing the relationship between the waterevaporated and the quantity q* can be obtained from Eq. (9) by settingthe quantity Δ equal to zero. Curve 104 in FIG. 9 shows the relationshipfor a standing pond.

In addition to the mass balance given above, consideration must also begiven to the balance of the pond as a whole. Considering the volume andsalt balance of the pond as a whole yields the following equations:

    q.sub.in =q.sub.e +q.sub.out +q.sub.d                      (10)

    q.sub.in S.sub.in =q.sub.d S.sub.d                         (11)

where q_(in) is the rate at which water is added to the surface of thepond, q_(e) is the excess of water evaporated from the pond overprecipitation into the pond, q_(d) is the discharge of salty water fromthe pond, S_(in) is the salinity of the makeup water, and S_(d) is thesalinity of the water removed from the pond.

If it is assumed that the discharge takes place at the surface of thepond, then S_(d) =S_(in). Basically, q_(e) is determined by climaticconditions, and q_(out) is determined by climatic conditions and designconsiderations of the control chamber. Furthermore, S_(in) is determinedby the water resources available so that Eqs. (10) and (11) contain twounknowns which can be solved for. Thus, the makeup water is as follows:

    q.sub.in =(q.sub.e +q.sub.out)[S.sub.d /(S.sub.d -S.sub.in)](12)

For S_(in) =zero, which is to say that fresh water is added, q_(in)=q_(e) +q_(out) and q_(i) =zero. Assuming that S_(in) =0.1, while thesalinity of the surface water S₁ =0.2, then q_(in) =2(q_(e) +q_(out))and q₀ =1/2q₁.

From the above, it can be seen that the control chamber shown in FIG. 6provides a convenient and relatively simple way in which to maintain thehalocline in a solar pond in any of the configurations shown in FIG. 4.Furthermore, the arrangement described above, by reason of the control83, permits the nature of the halocline to be changed over a long periodof time. For example, as explained previously, the halocline associatedwith a descending solar pond has a shape which is not stable in thepresence of considerable wind mixing at the surface of the pond becauseof the shallow slope of the halocline near the surface. This profile canbe corrected by altering the flow of water into the control chamber.

It is presently estimated that the area of the control chamber withrespect to the area of the pond should be approximately 1:100. With thisarrangement, the halocline will be stable over extremely long periods oftime, permitting the maximum amount of heat to be extracted on acontinuous basis by means of a heat exchanger as indicated by referencenumeral 73 in FIG. 6.

Another embodiment of a control chamber for maintaining the desiredfunctional relationship between the salinity in the halocline and thedepth of the halocline is shown in FIG. 8. Reference numeral 70Adesignates a solar pond according to the present invention into whichcontrol chamber 71A in the form of a flash evaporator is incorporatedfor the purpose of eliminating the necessity for pumping hot brinebetween the pond and flash evaporator and for permitting all of the heatwhich can be taken from the heat storage layer on a steady state basisto be removed from the pond in terms of the latent heat of the waterevaporated in the flash evaporator. The significance of these twofactors can be appreciated by considering U.S. Pat. No. 3,372,691 whichdiscloses a flash evaporator in combination with a solar pond, but whichrequires the use of pumps for transferring brine between the pond andthe heat exchanger and which will permit only a fraction of theavailable heat in the heat storage layer to be extracted in terms of thelatent heat of the water evaporated in the flash evaporator.

Referring now to FIG. 8, the pond is the same as shown in FIG. 6, andcontrol chamber 71A defines local region 82A which functions toevaporate water from the pond thereby increasing the salinity in theregion. Specifically, chamber 71A includes tube 84A verticallypositioned in the pond, the lower end of the tube being open at 86 at anelevation below the halocline. The upper end of the tube is closed bycap 88A and a vent 89 in the closed end of the tube provides an exitchamber for water that evaporates from inside the tube. Inside chamber71A is a vertically disposed baffle 85A which separates inlet 86A fromdischarge aperture 87A.

Some of the water from mixed layer 74 is returned to the heat storagelayer as indicated by line 90. This quantity is designated as q₁ and isdetermined by control valve 83A, the setting of which determines thequantity of water discharged from the system q_(d). Makeup water isfurnished via pipe 91, this water being designated q_(in) and is equalto the sum of the water discharged by valve 83A, the quantity of wateradded to the heat storage layer and the quantity of water q_(e)evaporated from the pond.

The column of water 92 inside tube 71A is denser than the water in heatstorage layer 80. This situation arises because water from heat storagelayer enters opening 86 on one side of baffle 85, flows upwardly towardsthe interface of the vapor and water in tube 71A, and then flowsdownwardly along the other side of the baffle, finally exiting throughopening 87A. The pressure P_(V) inside the flash evaporator is belowatmospheric pressure P_(A), and as a consequence the water at thesurface of the flash evaporator flashes into steam and exits throughvent 89A. Because water is constantly being evaporated from column 92,the density of the water passing through exit 87A will be greater thanthe density of water entering opening 86.

The height of the water in the flash evaporator above the level of thepond is designated h and is determined as follows:

    h=[1/(gρ.sub.1)][(P.sub.A -P.sub.V)-(D/ρ)(ρ.sub.1 -ρ)](13)

where ρ₁ is the density of the water flowing into the flash evaporator,ρ is the average density of the pond above the depth D, where D is thedistance between the level of the pond and the opening 86A in the flashevaporator, P_(A) is the atmospheric density in Pascal and P_(V) is thevapor pressure inside the flash evaporator.

The difference in density between the liquid leaving the flashevaporator and the liquid entering establishes the flow velocity at theinlet and exit of the flash evaporator. If the density of the outflow ofthe flash evaporator is ρ₂, then the velocities at the inlet and outletare as follows:

    V.sub.2 ≐V.sub.1 =[(2g)(h+D)(ρ.sub.2 -ρ.sub.1)/ρ.sub.2 ].sup.1/2                                                 (14)

As discussed above, where the heat flux in the heat storage layer isabout 50 watts per square meter of area of the pond, and assuming thatall of this heat is taken out in terms of the latent heat of the waterevaporated from the flash evaporator, then q₀ =2 Kg/day/m². If it isdesired for the discharged water to be about 3 degrees Celcius below thetemperature of the water in the flash evaporator, then the quantity q.will be approximately 200 times larger.

In such case, the velocity of the brine entering and leaving the flashevaporator will be approximately 0.4 m/sec. The area of the inlet andoutlet apertures in the flash evaporator will be approximately 10⁻⁵ m²for each square meter of the pond. Thus, for a pond that is 10,000sq.m., the openings should be approximately 0.1 sq.m.

FIG. 8 shows, in schematic form, heat exchanger 93 which extracts thelatent heat from the vapor evaporated by the flash evaporator producingat the end, slightly cooler fresh water. The heat exchanger is shownschematically and could represent a process steam line in amanufacturing plant, the exhaust steam being condensed and providingfresh water. This water will generally be valuable in an aridenvironment so that it would normally not be inserted back into the pondbut would provide a useful byproduct of the conversion of heat in thesolar pond into process steam for a manufacturing operation.

One of the corollary advantages in the arrangements shown in FIG. 8 liesin the ease with which the vacuum within the flash evaporator can bemaintained. This situation arises because the flash evaporator handleshot brine solutions which can contain only a very minute amount ofdissolved gases; and this reduces the amount of work required inmaintaining the vacuum.

The configuration of the flash evaporator shown in FIG. 8 will eliminatethe necessity for pumping hot brine between the pond and the flashevaporator and thus reduce the overall power requirements of the system.However, the application to the heat storage layer of the quantity ofwater designated q₁ achieves significant results independently of thesavings in power to be realized by utilizing the flash evaporator andconfiguration shown in FIG. 8. These results can be demonstrated byconsidering curve 103 and curve 104 in FIG. 9. Curve 103 represents afalling solar pond which can be achieved by combining a flash evaporatorwith a solar pond and evaporating a quantity of water from the flashevaporator that will just be the adequate to maintain the profile of thesalinity in the halocline. Where the salinity of the water at the bottomof the halocline is approximately 4 times the salinity of the water atthe top of the halocline, curve 103 shows that the ratio q₀ /q* isapproximately 0.8. This means that only about 10% of the availableenergy contained in the water in the heat storage layer can be extractedfrom the water evaporated from the flash evaporator in terms of itslatent heat. The remaining 90% of the heat in the heat storage layerwill have to be extracted by a heat exchanger such as indicated at 73Ain FIG. 8. In other words, no more than 10% of the available heat in theheat storage layer can be extracted from the latent heat in the waterevaporated from the flash evaporator if the halocline is to bemaintained. Evaporation of more water than this amount can beaccomplished, but only at the expense of degradation in the salinityprofile in the halocline. Therefore, this approach is not feasible overa long period of time because the pond will be destroyed. If the pond isto furnish processed steam, for example, at a given rate, the area ofthe pond will have to be considerably larger than will be the case whereall of the heat in the heat storage layer extracted in terms of thelatent heat of the water evaporated by the flash evaporator.

Considering now curve 104 which is for standing pond in which thequantity q is greater than zero, a 4 to 1 salinity ratio across thehalocline will result in the quantity q₀ /q* having a value ofapproximately 3. This approximately a 4-fold increase in the quantity ofwater that can be extracted by evaporation from the flash evaporatorwhile still maintaining the halocline. Furthermore, the halocline willbe a linear one in the case of the standing pond as compared to theconcave profile associated with a falling pond.

Consider now curve 101 for an ascending pond where the quantity q isapproximately 25% greater than the water evaporated from the flashevaporator. Inspection of the curves in FIG. 9 reveals that the quantityq₀ /q* for this situation is approximately 11 which means that all ofthe heat in the heat storage layer can be extracted on a steady statebasis in terms of the latent heat of the water evaporated from the flashevaporator. Furthermore, as indicated previously, the halocline for anascending solar pond will be convex in shape which will provide agreater resistance to wind-mixing at the surface.

It is believed that the advantages and improved results furnished by theapparatus of the present invention are apparent from the foregoingdescription of the several embodiments of the invention. Various changesand modifications may be made without departing from the spirit andscope of the invention as sought to be defined in the claims thatfollow.

What is claimed is:
 1. A method for producing power from an open body ofwater containing dissolved salts comprising:(a) establishing a haloclinein the body of water adjacent the surface thereof and above a heatstorage layer which is itself above the hypolimnion whereby solarradiation incident on the surface of the body of water is absorbed inthe halocline and transferred into the heat storage layer; (b) usingheat from the heat storage layer to operate a power plant having a heatengine and a condenser; (c) cooling the condenser with liquid drawn fromthe hypolimnion; (d) cooling the heated liquid from the condenser in acooling pond; and (e) returning liquid in the cooling pond to the bodyof water at a depth below the halocline.
 2. A method according to claim1 wherein the cooling pond is connected to the body of water only at thesurface thereof.
 3. A method according to claim 1 wherein heat isextracted from the heat storage layer by a heat exchanger locatedtherein.
 4. A method according to claim 1 wherein heat is extracted fromthe heat storage layer by drawing liquid therefrom into a heat exchangerand then discharging the cooled liquid into the body of water.
 5. Apower plant having, in combination, a heat source in the form of an openbody of water containing dissolved salts, a heat exchanger forextracting heat from the heat source and supplying heat to a heatengine, and a condenser for cooling the exhaust of the heat engine, theheat source comprising:(a) a halocline in the body of water adjacent thesurface thereof and above a heat storage layer which is itself above thehypolimnion whereby solar radiation incident on the surface of the bodyof water is absorbed in the halocline and transferred into the heatstorage layer; (b) means for drawing liquid from the body of water andincreasing its density to a value greater than the density of liquidbelow the halocline; and (c) means to inject the liquid of increaseddensity into the body of water below the halocline for inducingupwelling in the body of water thereby countering the effects ofmolecular diffusion of salt across the halocline.
 6. A power plantaccording to claim 5 wherein said heat exchanger extracts heat from theheat storage layer to drive the heat engine.
 7. A power plant accordingto claim 6 wherein the heat exchanger is located in the heat storagelayer.
 8. A power plant according to claim 6 wherein the heat exchangeris located remotely from the heat storage layer and liquid from thelatter is pumped into the heat exchanger.
 9. A power plant according toclaim 5 wherein said means for drawing liquid from the body of waterincludes an evaporating pond connected only to the surface liquid of thebody whereby the density of the liquid in the pond is increased byevaporation of water from the pond.
 10. A power plant according to claim5 wherein said means for drawing liquid from the body of water includesmeans for cooling the condenser with liquid drawn from the hypolimnionthereby producing a heated liquid, and means for discharging the heatedliquid into a cooling pond where the density of the heated liquidincreases as it cools.
 11. A power plant according to claim 5 whereinthe liquid drawn from the body of water is drawn from adjacent thesurface thereof.
 12. Apparatus comprising:(a) a halocline adjacent tothe surface of a solar pond for absorbing solar radiation, the salinityof the halocine having a predetermined functional relationship withdepth; (b) a heat storage layer below the halocline for storing heatfrom the halocline at a given average rate; (c) means for extractingsensible heat from the heat storage layer at substantially said givenaverage rate; and (d) means for maintaining the salinity in thehalocline at substantially said predetermined functional relationshipwith the depth; (e) said last-named means including means for inducingan upward vertical flow across the halocline to counter the effects ofwind-mixing and molecular diffusion.
 13. Apparatus in accordance withclaim 12 wherein the means for extracting heat include a flashevaporator.
 14. Apparatus in accordance with claim 13 whereinsubstantially all of the heat extracted from the heat storage layer iscontained in the latent heat of the water evaporated by the flashevaporator.
 15. Apparatus in accordance with claim 14 wherein water fromthe pond is added to the heat storage layer for maintaining thefunctional relationship of salinity with depth in the halocline. 16.Apparatus according to claim 15 wherein the added water is from a mixedlayer above the halocline.
 17. Apparatus according to claim 14 whereinthe added water is a portion of the water evaporated in the flashevaporator.
 18. Apparatus according to claim 13 wherein some of the heatextracted from the heat storage layer is by way of a heat exchangerassociated with the heat storage layer, and some by way of the flashevaporator.
 19. Apparatus according to claim 12 wherein the means forextracting heat includes a heat exchanger associated with the heatstorage layer.
 20. Apparatus according to claim 12 wherein saidlast-named means includes means for inducing a downward vertical flowacross the halocline whereby the salinity profile of the halocline issubstantially linear.
 21. Apparatus according to claim 12 wherein saidmeans for inducing an upward vertical flow includes means forevaporating water drawn from adjacent the surface of the pond andproducing concentrated brine, and means for injecting said concentratedbrine into the solar pond below the halocline.